Numerous polymeric compositions have been used as alloplastic implants in mammals for the following purposes:
(1) to augment tissue deficiency (e.g. in reconstructive facial surgery, mammary augmentation, etc.);
(2) to replace a functional body component, such as articular cartilage, bone, joint, blood vessel, trachea, dura matter, intraocular lens, etc.;
(3) to connect severed tissue (e.g. sutures and staples);
(4) to block certain ducts or passages (as in reversible sterilization by the Aldrich procedure); and
(5) to provide a depot for sustained and controlled drug delivery, both local and systematic.
Hydrogels, or the water-swellable but water-insoluble polymers, are among the polymeric substrates which have been used as implantable plastics since they offer, inter alia, certain advantages, e.g. permeability for water-soluble compounds, good biological tolerance and controllable softness and flexibility.
Polymeric implants are formed in defined shapes and sizes as demanded by the specific application. Such preshaped forms have several disadvantages. Most significantly, the applications require surgical procedures. Secondly, a polymeric implant, as a rule, is encapsulated by collagenous tissue capsule and is not integratable into surrounding tissue; porous polymeric implants may be connected to surrounding tissue by in-growing cells, however, such porous polymeric implants are often prone to calcification, etc. Thirdly, the size and shape of the polymeric implant must be determined in advance and any changes of size and shape are affected by re-implantation.
Initial attempts to utilize solidifying compositions were directed to applications, such as dental filling materials, acrylic bone cements, and tissue adhesives based on polyurethanes and alpha-cyanoacrylates. Subsequently, research was directed to solidifying reactive systems. Initially, soft polysiloxane gels were used which do not solidify but which are, to some extent, injectable while preserving some gel-like properties. Such polysiloxane gels are maintained in place by granulation of the surrounding tissue rather than solidification and suffer from the disadvantages of containing low-molecular weight compounds, such as cyclic oligomers, which slowly migrate into the organism.
More recent attempts regarding solidifying compositions utilized an aqueous solution of purified bovine collagens which are injected into the tissue and which gelatinate when heated above 37.degree. C. by collagen transition into fibrilar form without presence of low molecular weight toxic components. Collagen itself, however, is reactive and is gradually degraded by enzymatic reaction and is resorbed after a period of time. Thus, such a gelatinous implant is only a temporary replacement of a pre-formed implant which may be subsequently replaced by natural tissue. There is little control over such natural replacement, and unsatisfactory results occur in many instances.